The invention relates to a measuring apparatus and to a method of determining the flow of a fluid and to the use of the measuring apparatus.
Flow measuring systems for the determination of the flow of a fluid, for example through a line or a pipe, are known for different applications in different embodiments. To determine the flow of fluids of low density, in particular of gases, apparatuses are widely used which have a vertical flow pipe through which the gas flows from the bottom to the top and which has a suspended body which is held in a defined position in a state of equilibrium between the gravitational force acting on the suspended body and the upwardly acting force of the gas flowing past it, with the position of equilibrium of the floating body in the flow pipe being a measure for the amount of gas flowing through per time unit. In this connection, such flow measuring systems are frequently only designed with a visual display without a transmission function and cannot be operated with a horizontally aligned flow pipe.
Other known systems work with an orifice and a resistor of truncated cone shape which is arranged in front of the orifice in a hollow space, with the hollow space having an inflow opening and an outflow opening and with a fluid flowing through it whose flow should be determined. In this connection, the flow rate of the fluid should be determined from the difference between the pressures which are measured in front of the orifice and behind the orifice and from the position of the resistor. The size of the flow opening of the orifice is variable such that the pressure differential, which drops over the orifice, can be kept constant by means of a control unit.
Furthermore, flow measuring systems are known in which a movably arranged measuring element is acted upon by a pressure differential of a fluid flowing through a hollow body, with a force being exerted onto the measuring element such that the measuring element is stabilized in a fixedly pre-set zero position independently of the flow of the fluid. In EP 0 101 815 A1, a flow measuring system is shown comprising a venturi and a measuring device which includes a magnetic measuring element movably mounted in a hollow cylinder and connected to the venturi. The measuring element is acted upon by a pressure differential built up by the fluid flowing through the venturi and is thereby subjected to a deflecting force, with the measuring element preferably being a freely movable, permanently magnetic piston with which a light barrier is associated as a position sensor. In this connection, the measuring element is stabilized in a zero position with the aid of a control unit which cooperates with the position sensor to determine the position of the measuring element by a positionally independent magnetic restoring force, which is produced by an electromagnet.
However, this arrangement has a series of disadvantages. To realize a stable zero position of the measuring element, the magnetic restoring force in the flow measuring system disclosed in EP 0 101 815 A1 must be at least partly compensated by a second counter-force acting in the opposite direction. This counter-force can, for example, be the weight of the measuring element. For this purpose, the measuring device is arranged such that the measuring element can move vertically and is partly drawn in upwardly into the coil. The stable zero position, that is the position of the measuring element without a flow of the fluid through the venturi, is then where the magnetic force on the measuring element is opposite and equal to the weight of the measuring element. This means that this arrangement cannot be operated with a horizontal position of the hollow cylinder in which the measuring element is arranged. If, by a suitable selection of the mean density of the measuring element, its weight is set opposite and equal to the buoyancy forces in the fluid, the arrangement can admittedly be operated independently of the position of the hollow cylinder, but the counter-force required to stabilize the zero position must be produced by an additional electromagnet. The arrangement thus becomes a lot more complex and expensive since not only an additional electromagnet, but also the control units required for this have to be installed. In this connection, this arrangement is generally relatively complex and expensive in its design due to the use of a venturi with a separate measuring device. The measuring element, which is acted upon to measure the flow of the fluid from the pressure differential of the venturi, must be arranged in a hollow cylinder separate from the venturi. This has the consequence that the measuring device has to be connected to the venturi by additional connection means, for example by pressure lines. Furthermore, this flow measuring system only provides reliable measurement results when the flow of the fluid through the venturi is proportional to the output signal of the control unit, that is only when, in accordance with Bernoulli's equation, the differential pressure in the venturi is proportional to the square of the flow of the fluid. This simple relationship is, however, only rarely present and in practice much more complicated relationships frequently occur between the flow of the fluid and the pressure differential which occurs. As a rule, the pressure differential occurring in the venturi depends in particular on different parameters of the liquid such as on the viscosity or on the density so that the specific functional relationship between flow and pressure differential can also depend, among other things, on the type of liquid to be measured.
It is therefore the object of the invention to provide an improved measuring apparatus to determine the flow of a fluid in which the disadvantages known from the prior art have been avoided.
In accordance with the invention, a measuring apparatus is thus proposed to determine the flow of a fluid which includes a hollow body which has an inflow and an outflow (an inlet and an outlet) for the fluid. A magnetic sample is movably arranged in the hollow body and can be acted upon with a working pressure by the fluid. The measuring apparatus further includes at least one position sensor to determine the position of the magnetic sample, an electromagnet to exert a magnetic force on the magnetic sample and a control device for the electromagnet which is connected signal-wise to the position sensor, with the control device including regulating means which control the electromagnet with the help of a signal of the position sensor such that the magnetic sample is held in a position against the working pressure. The sample is designed and arranged such that the fluid can flow through the hollow body from the inflow to the outflow.
In a preferred embodiment of the measuring apparatus in accordance with the invention, the magnetic sample is arranged with respect to the electromagnetic in the hollow body such that a stable zero position of the sample can be realized solely by the electromagnet which, by exerting a magnetic force on the sample, can hold this in a pre-settable position against the working pressure of the flowing fluid. This means that, to stabilize the sample in the zero position, that is in that position which the sample adopts without flow of the fluid through the hollow body, no second force such as the weight of the sample or a second magnetic force is required which acts opposite to the magnetic force of the electromagnet. This can be achieved in that the magnetic sample in the zero position is correspondingly oriented in the axial direction with respect to the field line profile of the electromagnet and is symmetrically arranged. A restoring force always acts on the sample in the direction towards the zero position independently of the direction of an axial deflection.
In accordance with the invention, the flow through the hollow body is determined in that the magnetic sample is held in a position against the working pressure by the magnetic force of the electromagnet, with the electromagnet being able to cooperate with the sample such that no substantial movements of the sample take place with respect to the hollow body in the operating state on a change in the flow. This is in particular of advantage when, for example, the flow of a liquid which is prone to particle formation, or the flow of an aggressive liquid such as a polishing liquid, should be determined. Since the sample only moves in a low amount with respect to the hollow body, wear by friction effects is minimized, with the sample, in a special variant, also being able to change its position on a change of the flow, with the flow then being able to be determined from the change of position.
An important characteristic of the invention consists of the fact that the sample is designed and the hollow body is arranged such that flowing fluid flows around or flows through the sample and it is thereby acted upon by a working pressure which is a measure for the flow of the fluid through the hollow body. A separate measuring device outside the hollow body through which fluid flows, such as is known from the prior art, can thereby be dispensed with.
In a preferred embodiment of a measuring apparatus in accordance with the invention to determine the flow of a fluid, the hollow body has a circular flow cross-section and is designed, for example, in the form of a cylindrical pipe. A preferably plate-shaped magnetic sample is arranged inside the hollow body such that an axis thereof is movable in the axial direction along a longitudinal axis of the hollow body. The magnetic sample can, for example, include a plate-shaped, or ring-shaped, permanent magnet which, depending on the embodiment, can be polarized in the radial or in the axial direction. The sample can in particular also include two magnets magnetized in opposite directions in the axial direction, with such samples being termed within the context of this application as “axially oppositely magnetized samples”.
The sample can also, for example, include one or more bar magnets polarized in the direction of the body axis and/or perpendicular thereto or a differently shaped magnet. The sample can furthermore also include, for the direct diversion of the magnetic flow, ferromagnetic components, in particular iron parts in the form of plates or rings. If the magnetic sample substantially includes one or more ferromagnetic, non-permanently magnetic materials such as iron, this is in particular of advantage if the fluid is subject to larger temperature fluctuations. The sample can even include a second electromagnet fed from an electrical energy source, the second electromagnet being fed with electrical energy via feeders which are integrated into possibly present securing means for the sample. In special cases, it is even possible for the sample itself to include the electrical source of source of energy and/or for a combination of the previously mentioned types of magnetic samples to be used.
The hollow body further includes an electromagnet which is suitable to exert a magnetic force on the sample in the substantially axial direction. The electromagnet can include one or more coils which are suitably arranged in a manner known per se in the peripheral direction at the outside of the hollow body or in the wall of the hollow body. When the electromagnetic is stimulated by an electric current, the electromagnetic can enter into cooperation with the magnetic sample and exert a Lorentz force {right arrow over (F)}={right arrow over (B)}×{right arrow over (I)} on this, where, as customary, {right arrow over (B)} designates the magnetic flux density and {right arrow over (I)} designates the electric current through the coil of the electromagnet. In a special embodiment of the measuring apparatus in accordance with the invention, the magnetic sample, which can be magnetized, for example, substantially in the axial direction, can be symmetrically arranged in the axial direction in the operating state in the zero position with respect to the field line profile of the electromagnet and be oriented such that the sample is always subjected to a restoring force in the direction of the zero position with respect to an axial deflection. This means that the zero position of the sample is a stable position of equilibrium with respect to an axial deflection, with—in the context of this application—the zero position being understood as that position in the hollow body which the sample adopts under the effect of a magnetic force of the electromagnet when the flow rate of the fluid through the hollow body disappears, that is when the flow of the fluid through the hollow body is zero.
In another embodiment, the zero position of the sample can be freely selected by the regulation irrespective of the position of the sample with respect to the electromagnet, i.e. the zero position can be individually fixed for each measurement. The magnetic sample can advantageously be magnetized in the radial direction, with the force which the electromagnet exerts on the magnetic sample in the operating state being directly proportional in a good approximation to the flow through the electromagnetic at least with small deflections from a pre-settable zero position.
The position of the sample in the hollow body can be determined by at least one position sensor which is preferably arranged at the outside of or in the wall of the hollow body, especially also inside the hollow body, such that the position sensor can detect a position and/or a change in the position of the sample. The position sensor is connected signal-wise to a control device, with the control device including regulating means which control the electromagnet with the aid of a signal of the position sensor such that the magnetic sample is held in a pre-settable position, preferably in its zero position, against the working pressure of the flowing fluid. The control device can include an evaluation unit which, with the aid of the electrical energy required for the excitation of the electromagnet, in particular of the electrical current or of the voltage, determines, and emits via an output unit, the flow of the fluid through the hollow body. The output unit can be connected, for example, to a display apparatus to display the flow or with a memory unit to store the flow data. To determine the flow of the fluid, the control device can include a one-dimensional, or a multi-dimensional, look-up table which suitably links the associated value of the controlled variable, that is for example the electrical current through the magnet, with further data. Such a look-up table can be represented by a two-dimensional, or multi-dimensional, data field which, while taking into account different parameters such as the temperature or the pressure of the fluid and/or of different characteristic parameters of the fluid such as density, viscosity or other characteristic parameters such as geometric characteristic parameters of the measuring apparatus, associates the associated value of the flow of the fluid through the hollow body with each value of the current which has to be fed into the electromagnet, for example, with a given flow, to stabilize the zero position of the sample. It is also possible in specific cases to determine the flow amount without using a look-up table, for example directly from the electrical current through the magnet. The control unit can in particular also be suitable to control and/or to regulate further devices, such as a pump for the fluid, in dependence on the flow.
In accordance with the invention, the sample is designed and arranged such that the fluid can flow from the inflow to the outflow of the hollow body, that is the fluid can flow around the sample either along a surface of the sample between the sample and the hollow body or the sample has at least one bore through which the fluid can flow in the operating state. In a special embodiment of the measuring device, the fluid can also flow from the inflow to the outflow of the hollow body via an overflow line. The magnetic sample can have a jacket for protection against chemically and/or physically aggressive fluids such as acids or lyes, or mechanically aggressive polishing liquids in that it is cast, for example, in a resistant material or is accommodated in a protective sleeve in which the sample is arranged in a correspondingly freely movable manner.
If the sample has a bore through which fluid flows in the operating state, the outer shape of the sample can be designed such that the sample cooperates in an axially freely movable manner and in a shape matched manner with the interior of the hollow body. The hollow body thus preferably has a circular flow cross-section, with the sample being designed in a plate shape or in cylindrical shape with a circular cross-section. Every other shape-matched embodiment of hollow body and sample is also feasible, provided that the sample remains axially movable. The sample does not need to cooperate in a shape matched manner with the interior space of the hollow body, but can have any outer shape which ensures a sufficient axial freedom of movement. The sample can in particular, for example at the bore, also have an orifice which can be replaceable so that the measuring apparatus can be matched to the respective fluid and/or a preferred measuring range for the flow amount of the fluid. The viscosity dependence of the working pressure on the sample can in particular be minimized by the suitable use of an orifice with a given flow, so that changes in the viscosity of the fluid to be measured only insignificantly influence the result of the flow measurement during a measuring process.
If the sample has no bore through which the fluid can flow from the inflow to the outflow of the hollow body in the operating state, the sample is designed and arranged in the hollow body such that the fluid can flow around the sample, at least when the sample is located in its zero position. The sample can thus have cut-outs, for example, at its outer edge which form a passage for the fluid along the body axis such that the fluid can flow from the inflow to the outflow between the sample and the inner wall of the hollow body. The inner wall of the hollow cylinder can also have corresponding cut-outs through which the fluid can flow around the sample. In certain cases, if technical flow requirements make it necessary, for example, any combination of the aforesaid embodiments can also be realized in order to allow the fluid to flow through and/or flow past the sample. Generally, both the cross-section of the hollow body and the shape of the sample can have any desired design if the axial freedom of movement of the sample in the magnetic field of the electromagnet is ensured.
The samples can thus, for example, have a cigar shape, with the maximum diameter being selected such that the sample still has a certain radial freedom of movement. This can, for example, be of advantage if liquids flow through the hollow body which are prone to easy clumping in narrow intermediate spaces or, for example with mechanically aggressive polishing liquids which can result in corresponding damage to the measuring apparatus during flowing in narrow intermediate spaces between the sample and the hollow body. Biological fluids, such as blood, can in particular be prone to forming their own surfaces of a certain thickness at surfaces which are flowed about, for which space must correspondingly be available.
If the sample still has a certain radial freedom of movement, the measuring device can include securing means for stabilization, in particular of the radial position of the sample. The sample can thus, for example, be fixed with suitable threads to the inner wall of the hollow body or be suspended at bending beams which have a high transversal flexibility and thereby ensure a sufficiently high axial movability of the sample. Or, for example, a bead can be provided which has a spring effect which is as constant as possible in the flow direction. Different securing means are also possible. For example, the sample can also be stabilized and/or fixed magnetically or in another manner, in particular radially.
In particular when large flow rates of the fluid through the hollow body have to be realized, the hollow body can include a separate overflow line for the fluid which connects the inflow of the hollow body to the outflow of the hollow body, with the overflow line having a narrowing at which the fluid causes a pressure differential in a manner known per se with which the sample is acted upon. Since, in this arrangement, the fluid can flow through the overflow line from the inflow to the outflow of the hollow body, the sample can be designed in such a shape matched manner and be axially movable arranged in the hollow body such that the fluid can neither flow around nor flow through the hollow body. However, the sample can also be designed and arranged with this embodiment such that, as described above, the fluid can flow around the sample and/or through a bore.
Generally, with the measuring apparatus in accordance with the invention, the orientation of the direction in which the sample is movably arranged can adopt any desired angle with respect to the direction of gravity. However, if—in addition to the magnetic force of the electromagnet and to the force the sample is subjected to by being acted upon with the working pressure—yet another gravitational component acts on the sample, this must likewise be taken into account to determine the correct flow of the fluid. However, by a suitable selection of the mean density of the sample, its weight can be set opposite and equal to the buoyancies in the fluid, so that the measuring apparatus can be operated independently of the direction of movement of the sample with respect to gravity and the weight of the sample no longer needs to be taken into account to determine the flow of the fluid.
In accordance with the invention, the measuring apparatus includes a control device for the electromagnet which is connected signal-wise to at least one position sensor to determine the position of the sample and wherein the control device includes regulating means which control the electromagnet with the aid of a signal of the position sensor such that the magnetic sample is held in a position, preferably in the zero position, against the working pressure, with the position sensor preferably being arranged at the outside of the hollow body or in the wall of the hollow body, such that the position sensor can detect a position and/or a change in the position of the sample, The position sensor can also be suitably arranged inside the hollow body. A magnetic field sensor, in particular a Hall probe, a differential field sensor, for example a GMR sensor (giant magnetoresistive sensor) or an LVDT (linear variable differential transformer) or an eddy current sensor can preferably be used as the position sensor. It is also possible to determine the position of the sample in another manner, e.g. by means of a light barrier with optical means or in another way.
At least two position sensors are preferably used in each case to determine the position of the magnetic sample. The position sensors are connected in the manner of a bridge circuit with the control unit and provide, in the event of a deflection of the sample from a pre-settable position, a differential signal to the control unit whose strength and sign is dependent on the magnitude and on the direction of the deflection. Two, preferably four, position sensors can in particular be arranged, for example, oppositely disposed at the hollow body such that the strength of the differential signal only depends on the axial displacement, whereas a radial displacement is not taken into account.
In this connection, the sample can additionally have one or more position transducers which cooperate with the position sensor to determine the position of the sample. The sensitivity of the measuring apparatus can be increased and a decoupling of the position sensor from the magnetic samples can be achieved by the use of position transducers in the sample. Depending on the kind of position sensors used, the position transducer can include permanently magnetic components, ferromagnetic components, electrically conductive components, optical components or other components.
If the sample is acted upon with a working pressure by fluid flowing through the hollow body, the sample is subjected to a force due to the pressure differential acting on it which deflects it substantially in the axial direction, with the force which acts on the sample depending on and increasing with the flow of the fluid through the hollow body is. The energy supplied to the electromagnetic is regulated in dependence on the signal provided by the position sensor to the control unit such that the force which the magnetic field of the electromagnet exerts on the sample is just so large enough that the sample remains in a pre-settable position, preferably in its zero position, with the electric current which flows through the electromagnet being used as the preferred control variable to regulate the electromagnet. Different electrical operating parameters, such as the electrical voltage, can also be used as control variables.
In a preferred embodiment of the measuring apparatus in accordance with the invention, the regulating means include a PI controller (proportional plus integral controller) known per se, a known PID controller (proportional plus integral plus derivative controller) or a state controller and an electrical source of power to supply the electromagnet with electric power.
The control device can furthermore include an evaluation device which determines the flow of the fluid through the hollow body with the aid of the electrical energy required for the excitation of the electromagnetic, in particular the electrical current, and emits it via an evaluation unit. The evaluation unit can be connected, for example, to a display unit to display the flow or to a memory unit to store the flow data and, in special cases, cooperate with other regulating devices.
As a rule, the flow of the fluid through the hollow body is not in a simple relationship with the working pressure caused on the sample by the flowing fluid. The same thus applies to the relationship between the control variable, that is e.g. the electrical current through the electromagnet and the flow of the fluid. In special cases, the current with which the electromagnet has to be fed to stabilize the zero position of the sample is square to the flow of the fluid through the hollow body. This simple relationship is, however, frequently not realized in practice and be characteristically different depending on the kind of the liquid. Different characteristic values of the liquid such as the viscosity, the density or parameters such as the temperature of the fluid or geometrical factors can in particular also have a sensitive influence on the relationship between the control variable and the flow of the fluid.
To determine the flow, the associated value of the control variable, that is for example, the electric current through the magnet, must therefore be suitably linked with data which are made available in a data store, for example in the form of a so-called look-up table. Such a look-up table can be represented by a two-dimensional or a four-dimensional data field which, while taking into account different parameters such as temperature or pressure of the fluid and/or different characteristic values of the fluid such as density, viscosity or other characteristic values, for example geometrical factors, associates the associated value of the flow of the fluid through the hollow body with each value of the current which has to be fed into the electromagnet with a given flow to stabilize the zero position of the sample.
In a further embodiment of the measuring apparatus in accordance with the invention, the flow cross-section of the hollow body varies along the longitudinal axis and/or the cross-sectional area of the sample varies along the body axis in accordance with a pre-settable scheme. In a preferred variant, the flow cross-section of the hollow body expands conically, starting from the inflow in the direction of the outflow, from a first flow cross-section to a second flow cross-section and/or the flow cross-section of the hollow body narrows conically, starting from the inflow in the direction of the outflow, from a third flow cross-section to a fourth flow cross-section. The side of the sample facing the inflow expands correspondingly from a first diameter of a cross-sectional area of the sample to a second diameter of a cross-sectional area of the sample and/or a cross-sectional area of the sample narrows on a side of the sample facing the outflow such that the sample cooperates with the hollow body such that the flow depends on the spacing of the sample from the inflow or from the outflow respectively.
It is thus in particular possible to optimize the resolution of the measuring apparatus for special demands or to match it to special measuring regions for the flow. The pressure drop over the sample can thus, for example, be kept constant by setting a fixed value of the electrical current which feeds the electromagnet such that the flow of the fluid can be determined from the position of the sample, with the position of the zero position being able to be adapted individually for each measurement by the control device depending on the requirement, such that, for example, the gradient of a characteristic line which links the flow of the fluid with the associated current through the electromagnet is varied, whereby the measuring sensitivity can be changed. This means that, by a suitable choice of the zero position, one and the same measuring device can be configured for different measuring ranges of the flow and can even be adapted during the operation of the measuring range.
The sample is in particular designed such that the sample cooperates with the hollow body such that, by a suitable positioning of the sample, the flow of the fluid through the hollow body is completely prevented. The sample and/or the hollow body preferably has, as already described, a cut-out which forms a passage for the fluid along the body axis so that, in dependence on the position of the sample, the fluid can flow or not flow between the sample and the inner wall of the hollow body from the inflow to the outflow. To satisfy the same function, the sample can also have one or more bores. With this variant, the flow of the fluid through the hollow body can be determined as described above, on the one hand; the flow of the fluid can also be regulated, on the other hand, in that a force is exerted on the magnetic sample by the electromagnet such that the sample is brought into a pre-settable position and is held there.
In a preferred embodiment, the control apparatus includes a valve regulator which compares a pre-settable desired value for the flow of the fluid with an actual value determined from the control variable, for example from the magnitude of the electrical current through the electromagnet, so that the control unit controls the electromagnet such that the pre-set desired value is regulated To determine and regulate the flow of the fluid, a look-up table is preferably used which, as already stated, can be represented by a two-dimensional or a three-dimensional data field which while taking into account different parameters such as the temperature or the pressure of the fluid and/or different characteristic values of the fluid such as density, viscosity or other characteristic values, associates a value of the electrical current to a flow of the fluid through the hollow body, said current having to be fed into the electromagnet for the corresponding positioning of the sample. It is also possible with the aid of the look-up table to associate the associated flow of the fluid to a predetermined value of the electrical current and possibly to control and/or to regulate further devices such as a pump for the fluid in dependence on the flow.
Furthermore, the measuring apparatus can also be used to determine a characteristic value of the fluid, in particular the viscosity and/or the density. For this purpose, the sample is preferably held by the magnetic force of the electromagnet at a pre-settable position, for example in the zero position or at another position, with a constant flow. If, with a constant flow of the fluid, a characteristic value of the fluid, for example the viscosity, the density or another characteristic value changes in dependence on the time, generally the value of the electrical current which flows through the electromagnetic must be adapted correspondingly if the sample should maintain the pre-determined position in the hollow body. Analogously to the previously described method, when a corresponding look-up table is used with the given flow, the corresponding characteristic value and/or its change can be determined from the value and/or from the change of the electrical current through the electromagnet.
Since the measuring precision of the measuring apparatus also depends, among other things, on the flow of the fluid, a measuring apparatus with an adjustable measuring precision is available with the previously described embodiment. The measuring precision of the measuring apparatus can, for example, be increased in that the sample is held at a pre-settable position which corresponds to a higher flow. As a rule, the measuring precision falls correspondingly when the flow is reduced. The position of the sample can preferably be set such that, at a maximally tolerable pressure drop over the sample, an optimum resolution is achieved. The current through the electromagnet can in particular be set for a given measuring range of the flow, for example, to a constant value so that, with a changing flow, the pressure differential over the sample remains substantially constant and the flow of the fluid and/or the change of the flow can be determined from the position and/or from the change in the position of the sample.
The method in accordance with the invention to determine the flow of a fluid is carried out by means of a measuring apparatus which includes a hollow body which has an inflow and an outflow for the fluid and in which hollow body a magnetic sample is movably arranged. The measuring apparatus further includes an electromagnet to exert a magnetic force on the magnetic sample as well as a control unit for the electromagnet which is connected signal-wise to at least one position sensor to determine the position of the magnetic sample. With the method in accordance with the invention, the fluid flows through the hollow body from the inflow to the outflow such that the sample is acted upon by a working pressure. The electromagnet is controlled with the aid of a signal of the positional sensor such that the magnetic sample is held in a position against the working pressure.
Furthermore, the measuring apparatus is used in accordance with the invention to determine a characteristic value of a fluid, in particular the viscosity and/or the density and is used as a valve and/or as a flow regulating system for a fluid.
The invention will be described in more detail in the following with reference to the drawing.